(i) Field of the Invention
The present invention relates to a noise eliminating apparatus of a high-frequency transmission system, in which high-frequency signals are transmitted through an unbalanced line comprising a coaxial cable, for eliminating noise entering the coaxial cable from the outside of and flowing through the unbalanced line.
(ii) Description of the Related Art
In a conventional high-frequency transmission system such as a CATV system in which high-frequency signals such as television broadcasting signals are transmitted through an unbalanced line comprising a coaxial cable, noise generated around the coaxial cable constituting a transmission line sometimes enters the outer conductor and, in some cases, even the central conductor of the coaxial cable, and is transmitted to a terminal device as common mode noise with the high-frequency signals to be transmitted.
When the transmission signals including such noise are transmitted directly to a subscriber terminal device, there arise a problem, for example, that proper reproduced signals (image signals, sound signals and the like) cannot be obtained at the subscriber terminal device even if the transmission signals from the system are received and demodulated.
Conventionally, therefore, the coaxial cable constituting the high-frequency transmission line from the center equipment of the high-frequency transmission system to the subscriber terminal device, namely, the trunk, the branch line branched from the trunk toward the subscriber""s home, or the lead-in line to the subscriber""s home connected to the branched line through a protector is inserted in a ferrite core for eliminating noise. As a result, noise components flowing through the outer conductor of the coaxial cable is eliminated.
Specifically, since an outside noise enters the mesh-like outer conductor constituting the coaxial cable and only the noise components surpassing the shielding value of the coaxial cable further enters the central conductor, i.e. the core wire of the coaxial cable, transmission of the noise toward the subscriber terminal device is supposed to be prevented by eliminating the noise components flowing through the outer conductor of the coaxial cable using the ferrite core.
However, the noise components induced in the central conductor of the coaxial cable cannot be eliminated, while the noise components flowing through the outer conductor of the coaxial cable can be eliminated by using the ferrite core. That is, the common mode noise cannot completely be eliminated.
Wherefore, an object of the present invention is to provide a noise eliminating apparatus capable of completely eliminating a common mode noise resulting from an outside noise entering a coaxial cable constituting the transmission line of a high-frequency transmission system such as a CATV system.
To achieve this and other objects, there is provided, according to one aspect of the present invention, a noise eliminating apparatus connected to a pair of coaxial cables for high-frequency transmission through respective connecting terminals, wherein a pair of balance-to-unbalance transformers for transforming unbalanced lines comprising coaxial cables into balanced lines are connected to the respective terminals, and wherein balanced lines connecting the pair of balance-to-unbalance transformers each other are provided with a noise eliminating transformer for canceling noise components flowing through a pair of lines constituting the balanced lines.
In the noise eliminating apparatus of the present invention, when noise is inputted from one connecting terminal through the coaxial cable, noise components in the outer conductor and noise components in the central conductor cancel each other through the winding of the unbalanced line of the balance-to-unbalance transformer provided on the side of the connecting terminal from which the noise is inputted, with the result that the noise components in the central conductor are eliminated.
The noise flowing through the coaxial cable include noise components in the outer conductor generated by induction of the outside noise into the outer conductor of the coaxial cable and noise components in the central conductor generated by induction of the noise components surpassing the shielding value of the coaxial cable. Since the signal level of the noise components in the central conductor is lower than that of the outer conductor, these noise components cancel each other at the balance-to-unbalance transformer and the noise components in the central conductor can be completely eliminated.
The remaining noise components unable to be eliminated by the balance-to-unbalance transformer flow through the inside balanced lines. However, a noise eliminating transformer provided on the balanced lines for canceling common mode noise components flowing through lines constituting the balanced lines completely eliminates common mode noise inputted from the connecting terminal.
According to another aspect of the present invention, the noise eliminating apparatus comprises: an insulating transformer having a primary winding connected to the central conductor and the outer conductor of a coaxial cable constituting a system side transmission line and a secondary winding connected to the central conductor and the outer conductor of a coaxial cable constituting a subscriber side transmission line; and a noise eliminating transformer having a pair of windings provided on the signal paths between both ends of one winding of the insulating transformer and the central conductor and the outer conductor of the coaxial cable connected to the both ends of the winding, the noise eliminating transformer eliminating common mode noise components flowing through the signal paths.